Objective: There is little research on the ventricular electrical aspects following cardiac resynchronization therapy (CRT). This study sought to establish electrocardiographic criteria associated to the ventricular electrical activation process that could identify patients with functional class (FC) improvement after CRT, by using the noninvasive method of body surface potential mapping (BSPM). Methods: Fifty-six patients with chronic heart failure and left bundle-branch block (LBBB), who had undergone CRT, with mean age 59.9±10.8 years, left ventricular ejection fraction 30.9±8.3%, QRS 184±35ms, FC (NYHA) II – 16%, III – 68% and IV – 16%, were selected. Through the 87-lead BSPM isochronous maps, ventricular activation times (VAT) of both right (RV) and left ventricles (LV) were analysed, in two situations: (1) native LBBB and (2) during biventricular pacing. After CRT, patients were divided in two groups: with and without FC improvement. The VATs were compared by the Mann-Whitney’s test. The ratio of patients with and without FC improvement who showed RV-to-LV VAT difference ≤20ms, and >20ms, was compared using the Fisher’s test. Significance level was accepted as p≤0.05. Results: Clinical characteristics of patients before CRT were similar in the groups. Patients with FC improvement (47) had RV-to-LV VAT difference during biventricular pacing shorter than those without FC improvement (14.40±13.0ms vs 23.8±9.4ms, p=0.0151). Moreover, the majority of patients with FC improvement had an RV-to-LV VAT difference ≤20ms during biventricular pacing (70% vs 22%, RR 5.8, CI 95% 1.334–25.517, p=0.01). Conclusion: The RV-to-LV ventricular activation time difference of less than or equal to 20 milliseconds, as characterised by BSPM, could identify patients who presented with improved functional class after undergoing CRT.